French Patent Application No. 2,335,929, filed on Dec. 15, 1975, describes the arrangement of a pneumatic spring, also known as a gas spring, with a view to the emission of an electrical signal as soon as a determined geometrical condition of the device is reached, in the case described the extreme "rod retracted" position, said signal being capable of engaging or disengaging an associated electrical device for indication or warning, for example. As a result, a change of condition owing to a relative movement of the rod away from the said determined position enables any undesirable movement of a component associated with the pneumatic spring to be detected, such as the rear door or the hood of a motor vehicle, or a trap-door or other hinged access door.
However, said detectable condition of the geometry of the pneumatic spring does not necessarily correspond to a single point but may extend over a certain length of its travel, as is described in French Patent Application No. 2,500,964, filed on Feb. 27, 1981, which advocates the use of elastic strips which are in contact at one time with the conducting wall of the tube of the pneumatic spring and separated from the latter at another time.
The present invention relates to a pneumatic spring of this type and the invention proposes improvements to it concerning detection of position.